The present invention relates to an ink jet head for ejecting ink drops and performing printing with the ink drops on paper or the like, and an ink jet recording apparatus with such an ink jet head mounted thereon.
Recently, an ink jet recording apparatus has become required to have a very small actuator because it has become higher in printing speed and smaller in size due to its multi-nozzle structure. Therefore, there is an ink jet recording apparatus in which electrostatic power is used for an actuator (for example, JP-A-6-71882). In this ink jet recording apparatus, the actuator is constituted by parallel plate electrodes, so that the apparatus has a feature that the actuator can be miniaturized, and a multi-nozzle structure can be realized.
The summary of this ink jet head driven by an electrostatic actuator will be described on the basis of the sectional view of FIG. 12 and the plan view of FIG. 13. The ink jet head of FIGS. 12 and 13 has a stack structure in which an electrode glass substrate 100, a diaphragm substrate 200 and a nozzle plate 300 are alternately stacked one after another and bonded with each other. Ink 400 supplied to a reservoir 204 from an ink supply port 104 opened in the electrode glass substrate 100 is distributed equally to a plurality of cavities 203 through orifices 302. The lower surface of each cavity 203 is constituted by a transformable diaphragm 201 so as to face an individual electrode 101 through an insulating film 202 for preventing shortcircuit to thereby constitute an electrostatic actuator 50. A voltage is applied between the diaphragm 201 and the individual electrode 101 so that an electrostatic attractive force is generated to thereby transform the diaphragm 201 downward. Then, an ink drop 401 is ejected from a nozzle 301 by the pressure of a spring force of the diaphragm 201 generated when the applied voltage is removed.
However, in a direct driving system in which a voltage is applied to individual electrodes 101 directly, n wirings to individual electrode pads 102 and one wiring to a GND pad, that is, (n+1) wirings in total from a control circuit 2 are required when n electrostatic actuators (C1 to Cn) 50 are provided, as shown in the circuit diagram of FIG. 14. Therefore, not only the space for a wiring connection portion increases, but also it is difficult to ensure the reliability. Particularly, the electrostatic capacity of the electrostatic actuators 50 is very small so that it may couple with the electrostatic capacity of each of the individual wirings from the control circuit 2. Accordingly, there has been a possibility of generation of scattering in the electrical characteristics of the electrostatic actuators 50.
In addition, JP-A-5-31898 discloses an ink jet head in which, in order to avoid complex wiring, on a substrate having a plurality of exoergic resistance elements arranged thereon, functional elements, integrated circuits and contacts for connecting the substrate to the outside are formed on the substrate. However, this ink jet head adopts a driving method called a bubble jet system, and its configuration is different from that of the above-mentioned ink jet head having the electrostatic actuators. Therefore, it was impossible to apply the head disclosed in the above Publication, as it is, to an ink jet head adopting electrostatic actuators.
It is an object of the present invention to provide an ink jet head adopting electrostatic actuators in which the total number of wirings is reduced so that not only the reduction of the space of a wiring connection portion can be intended, but also the reliability can be ensured.
It is another object of the present invention to provide an ink jet head in which printing accuracy is improved in addition to the above-mentioned object.
It is a further object of the present invention to provide an ink jet recording apparatus mounted with the above-mentioned ink jet head mounted thereon.
(A) According to the present invention, provided is an ink jet head which comprises an ink jet head chip having a plurality of nozzle holes, a plurality of independent ejection chambers communicating with the nozzle holes respectively, diaphragms constituting at least one-side walls of the ejection chambers, and individual electrodes disposed so as to be opposite to the diaphragms through air gaps respectively, and a control circuit for applying voltages between the diaphragms and the electrodes to perform charging/discharging so as to transform the diaphragms so that ink drops are ejected from the nozzle holes, respectively, wherein at least a part of the control circuit is constituted by an integrated circuit, and provided in the ink jet head chip.
In this invention, the control circuit is provided in the ink jet head chip. Therefore, not only it is possible to reduce the space of the wiring connection portion, but also it is possible to prevent the electrical characteristics of the electrostatic actuators from scattering. Accordingly, also from these points, it is possible to ensure the reliability.
(B) Further, in the above ink jet head according to the present invention, a part of or a whole of the control circuit is provided on a substrate (nozzle substrate) in the ink jet head chip, the plurality of nozzle holes being formed on the substrate.
The reason why control circuit is provided on the nozzle substrate according to the present invention is as follows.
{circle around (1)} The nozzle substrate is preferable for manufacturing an integrated circuit, because the heating step is gentle although the substrate is subjected to the heating step.
{circle around (2)} The nozzle substrate and the diaphragm substrate can be bonded with a bonding agent, and there is no fear that the control circuit is broken.
{circle around (3)} Since any substrate is sufficient for the nozzle substrate so long as a nozzle hole is opened therein, there is less limitation about the thickness in the nozzle substrate, and an Si substrate of a standard thickness (400 to 500 xcexcm) may be used as the nozzle substrate.
In addition, when the control circuit is provided on the front surface (outside surface) of the nozzle substrate, there is an advantage that the availability of the substrate is high since a one-side mirror wafer can be used as the nozzle substrate.
In addition, when the control circuit is provided on the back surface (bonded surface) of the nozzle substrate, epoxy resin used for bonding the nozzle substrate with the diaphragm substrate may be used for a mold, so that no step is produced in the front surface of the nozzle substrate.
(C) Further, in the above ink jet head according to the present invention, a part of or a whole of the control circuit is provided on a substrate in the ink jet head chip, the diaphragms being formed on the substrate.
The reason why the control circuit is provided on the diaphragm substrate in the present invention is as follows.
{circle around (1)} Since the diaphragm substrate is formed of an Si single-crystal wafer, the control circuit can be built in on one and the same substrate.
{circle around (2)} All the steps other than the wet etching step, that is, the boron diffusion step, the thermal oxidation step, the patterning step of a thermally oxidized film, the electrode sputtering step, and so on, are common to each other between the cavity formation and the integrated circuit formation. Accordingly, it is possible to reduce the number of step.
Particularly, when the control circuit is provided on the back surface (on the electrode glass substrate side) of the diaphragm substrate, the mechanism becomes simple since a current can be applied to the individual electrode only by providing bumps or the like.
(D) Further, in the above ink jet head according to the present invention, a part of or a whole of the control circuit is provided on a substrate in the ink jet head chip, the individual electrodes being formed on the substrate.
When the control circuit is formed on the electrode substrate (glass substrate) according to the present invention, there are advantages as follows.
{circle around (1)} When a part of the control circuit is constituted by a TFT, the individual electrode and the control circuit can be manufactured on one and the same substrate by manufacturing the TFT on a neutral borosilicate glass through a passivation film, so that the connection between the individual electrode and the control circuit becomes easy.
{circle around (2)} The operation of the TFT can be confirmed by bringing a contact probe into contact with the individual electrode having a large area, so that inspection is easy.
(E) Further, in the above ink jet head according to the present invention, the control circuit has a resistor interposed in a charging path for each of electrostatic actuators constituted by the diaphragm and the individual electrode, and a resistor interposed in a discharging path for the electrostatic actuator, the value of the resistor interposed in the charging path being set to be larger than the value of the resistor interposed in the discharging path.
The time constant is increased by increasing the resistant value of the charging path, so that the electrostatic actuator can be driven gently. Accordingly, the electrostatic actuator can cope with the fluid resistance of an ink supply system. In addition, the time constant is reduced by reducing the resistant value of the charging path, so that the electrostatic actuator can be driven suddenly. A stable operation can be obtained by driving the electrostatic actuator in a manner as described above so that high resolution printing can be obtained.
(F) Further, in the above ink jet head according to the present invention, the control circuit switches the direction of charging for each electrostatic actuator constituted by the diaphragm and the individual electrode between the forward and backward directions alternately so as to make the ink jet head eject an ink drop twice for every dot. For example, the control circuit has switching elements connected in a bridge form to the each electrostatic actuator, and controls the open/close of the switching elements to thereby switch the direction of charging.
The quantity of ink to be ejected every time is reduced by ejecting an ink drop twice for every dot in such a manner, so that high resolution printing can be performed. In addition, since the direction of charging the electrostatic actuator is switched between the forward and backward directions alternately, residual charge after ejection is canceled, so that the relative displacement between the diaphragm and the electrode at the time of printing becomes stable. Also from this point, high resolution printing can be performed.
(G) In addition, an ink jet recording apparatus according to the present invention is provided with the above-mentioned ink jet head mounted thereon. Accordingly, an ink jet recording apparatus in which high quality printing can be performed is realized.